1. Field of the Invention
The present invention relates to a welding construction and a heat exchanger provide with the welding construction which are applied, for example, when a heat exchanger or the like for cooling natural gas by heat exchange is assembled.
2. Description of the Related Art
It is desired that for example, equipment such as a heat exchanger, piping, tank and the like introduced into a plant for natural gas processing are formed using an aluminum alloy excellent in mechanical properties and heat transfer in a low temperature service. Particularly, 5083 material defined in the Japanese Industrial Standard is said to be suitable as material for the above-describe equipment since the 5083 material has relatively high strength in a low temperature service among aluminum alloys.
Accordingly, in the past, an aluminum alloy of 5083 material is prepared as a welding object member having a predetermined shape by working such as extrusion or cutting. In some cases, thereafter, backing. metal is mounted on the wall surface on the side in contact with natural gas, and assembling and welding are done using welding metal formed of 5183 material from on the side of the other wall surface to form a welding part whereby the welding object members are joined through the welding part.
However, in the above-described constitution, it is sometimes that when welding takes place using welding metal, molten welding metal comes in contact with fluid from a clearance between the welding object member and the backing metal. Since the welding metal in contact with fluid is formed of 5083 material containing magnesium at a percentage content of, for example, not less than 2.0%, a large amount of magnesium precipitates in the form of xcex2 layer (Mg2Al3) in the cooling process. Accordingly, there is a problem that when equipment assembled in that state is used for natural gas processing, a thermal influence part caused by welding and much magnesium precipitated in welding metal come in direct contact with natural gas, because of which stress corrosion cracking caused by reaction between magnesium and mercury tends to occur.
It is therefore an object of the present invention to provide a welding construction and a heat exchanger provided with the welding construction capable of preventing the stress corrosion cracking caused by reaction between magnesium precipitated in welding metal and mercury.
For solving the above-described problem, according to the present invention, there is provided a welding construction for a welding object member applied to a joining part in contact with a processed fluid containing mercury and formed of an aluminum alloy containing magnesium, comprising: backing metal provided on the wall surface on the side with which the processed fluid in the welding object member comes in contact; a welding part formed by assembling and welding using first welding metal formed of an aluminum alloy containing magnesium at not less than a predetermined percentage content in a space part formed by the backing metal and the welding object member; and a seal part formed, so as to prevent the first welding metal from leaking out during assembling and welding from a contact part between the backing metal and the welding object member, by seal-welding the contact part using second welding metal formed of an aluminum alloy containing magnesium at not more than predetermined percentage content.
According to the above-described constitution, the seal part formed in the contact part between the backing metal and the welding object member is able to positively interdict the contact between the first welding metal and the processed fluid. Further, the seal part is formed of the second welding metal which is smaller in percentage content of magnesium than the first welding metal. Thereby, even in the state that when the seal part is formed, the second welding metal in the seal part leaks out on the processed fluid side into contact with the processed fluid, the precipitation amount of magnesium of the second welding metal is less than the first welding metal, thus making it possible to make occurrence of stress corrosion cracking caused by reaction between magnesium and mercury in the processed object hard.
Further, the backing metal is characterized by comprising an aluminum alloy containing magnesium at not more than predetermined percentage content. According to the above-described constitution, even if the backing metal should precipitate magnesium by heating at the time of assembling and welding, the precipitation amount of magnesium is reduced, making it possible to make occurrence of stress corrosion cracking hard.
Furthermore, the backing metal is formed so as to cover a heat affected zone where magnesium is precipitated in the welding object member by heating at the time of assembling and welding, whereby contact between magnesium precipitated in the heat affected zone and the processed fluid can be prevented to enable making occurrence of stress corrosion cracking further hard.
Moreover, the welding construction according to the present invention, since the precipitation of magnesium is suppressed by standardized heat treatment, stress corrosion cracking can be made further hard to occur.
It is desired that the welding object member and the welding part comprise an aluminum alloy containing magnesium at not less than 2.0% of percentage content, and the backing metal and the seal part comprise an aluminum alloy containing magnesium at not more than 2.0% of percentage content.
The aluminum alloy of the welding object member and the welding part can be selected out of various alloys classified as a 5000 system in the Japanese Industrial Standard, and the aluminum alloy of the backing metal and the seal part can be selected out of various alloys classified as 1000, 2000, 3000, 4000, 6000, and 7000 systems in the Japanese Industrial Standard. According to the above-described constitution, since standardized general aluminum alloys can be used for the welding construction, material costs can be reduced.
If the welding construction of the present invention is applied to joining parts of a heat exchanger, piping or tank, stress corrosion cracking in the joining parts of a heat exchanger, piping or tank can be prevented.
For example, there can be mentioned a heat exchanger formed by assembling and welding a header formed of an aluminum alloy in which precipitation of magnesium is suppressed by standardized heat treatment and an apparatus body and for heat exchanging natural gas containing mercury, comprising: backing metal provided on the inner wall surface of the header prior to the standardized heat treatment and placed in contact with the other welding object member at the time of assembling and welding; a welding part formed by the assembling and welding using first welding metal containing magnesium at not less than a predetermined percentage content in a space part formed by the backing metal, the header, and the apparatus body; and a seal part formed, so that the first welding metal does not come into contact with fluid in a contact part between the backing metal and the apparatus body, by seal-welding the contact part using second welding metal containing magnesium at a not more than predetermined percentage content.
According to the above-described constitution, since the seal part formed in the contact part between the backing metal and the apparatus body prevents leaking of the first welding metal at the time of assembling and welding, it is possible to positively prevent the contact between the first welding metal and natural gas. Further, the seal part is formed of the second welding metal which is smaller in percentage content of magnesium than the first welding metal. Thereby, even in the state that when the seal part is formed, the second welding metal of the seal part leaks out on the natural gas side into contact with the natural gas, since the precipitation amount of magnesium of the second welding metal is less than the first welding metal, it is possible to make occurrence of stress corrosion cracking caused by reaction between magnesium and mercury in natural gas hard.